1. Field
One embodiment of the invention relates to an amplitude shift keying (ASK) demodulator that demodulates an ASK signal wave modulated by ASK, a communication module, a communication device, and an ASK demodulation method.
2. Description of the Related Art
A general amplitude shift keying (ASK) demodulator that demodulates an input signal modulated by ASK (an ASK signal) is provided with a rectifier and a comparator. The rectifier detects and rectifies a signal received through an antenna, thereby obtaining a demodulated signal. The comparator compares the demodulated signal with a threshold and amplifies it to a logic level, thereby converting it to a binary signal. It is often the case that the comparator has the hysteresis function to suppress malfunction due to noise (see Japanese Patent Application Publication (KOKAI) No. 2006-295319).
Japanese Patent Application Publication (KOKAI) No. 2006-295319 discloses a conventional ASK demodulator. With the conventional ASK demodulator, even if a period of no signal or a period for a logic “0” or “1” in an input ASK signal continues for a long time, an output signal neither becomes unstable nor varies because of the comparator with the hysteresis function. Besides, the conventional ASK demodulator is not provided with a differentiating circuit inside it, and therefore has no problem in demodulating a band-limited ASK signal.
With the conventional ASK demodulator, however, the reception sensitivity is limited due to a DC offset voltage caused by element dispersions and hysteresis width of the comparator with the hysteresis function.
If the hysteresis width is set to narrow, the drop in the reception sensitivity can be reduced. To set the hysteresis width to narrow, it is necessary to increase the resistance ratio of the comparator. On the other hand, to improve the reception sensitivity by reducing the DC offset voltage caused by element dispersions, it is necessary to increase the size of the element. Such an increase in the resistance ratio and the size of the element results in higher costs, especially when considering integration into large scale integration (LSI).